no doubt

Ahhhh...now that makes sense. Thank you.


I'm going to break out my ohmmeter and take some basic engine-off measurements on the MAF now.

no doubt

MAF sensor resistance between pins 3 & 5: 4.2 ohms

Idle mixture resistance: 879 ohms (seems high!)

no doubt

OK, so I dialed the resistance down to 600 ohms per the JDS faq sheet (thanks for that link!) and my shark won't even start with the MAF connected at that setting. Back up to 980 ohms (the max that it can be set) and I get the crazy idle behavior.

Well, enough of that testing! The MAF might not be the *only* thing wrong with my 928 at the moment...but it's about to be replaced!

John Speake

The ilde mixture pot on the MAF isn't connected to anything on your car......... which has cats/O2 sensor.

WallyP

John,

Just curious-
Actually not connected, or just ignored due to the coding plug setting?

John Speake

Hi Wally,
Not connected to anything in S4 and later. Only 5 wires in the cable to the 6 way MAF connector.

On non cat cars S4 and later, there is a idle mixture pot mounted above the fuse/relay panel because it's impossible to adjust the pot on the MAF in situ.

In the pre 87 LH cars, the pot on the MAF is connected to the LH ECU and idle mixture is adjusted with it.

On S4 and later if the coding plug says "non cat car" then the LH expects to see a pot connected.

As you know, the pot isn't actually connected to anything inside the MAF - only ground at one end.

no doubt

OK. But is my problem fuel flow related (clogged fuel filter, dying fuel pump, injectors, FPR's, dampers) or MAF related?

I get a steady idle with the MAF disconnected. This is repeatable so I'm confident in saying that, however, the car has almost no power to move and won't rev above 2,000 rpms in this state.

In contrast, I can rev to 4,000 rpms (haven't tried to go higher while parked) with the MAF connected, but the motor won't stay revved...I can make it spike up to any given rpm level, but it immediately falls back into the idle range regardless of what I do with the gas pedal (e.g. holding it steady at a depressed position). With the MAF still connected, the idle will fluctuate wildly until I spray starter fluid into the air intake...then the idle is smooth.

Some part of the control system has failed.

I'm ordering a fuel pressure gauge today, but I'm still going to be wondering if this is an electronic signal issue failure, or if it is a physical fuel flow failure.

John Speake

It's difficult to say what the problem is without more information, but I would suspect MAF failure.

You can check fuel flow at the return at the top of the fuel tank, fuel is only returned if the FPRegs are getting sufficient pressure, so it's a good check.

You can access the return pipe under that bung in the trunk floor on the RHS, stick that into a suitable container, bridge the fuel pump and you should get about 2 pints worth in 30sec.

no doubt

First, thank you! Thanks to everyone here who added so many kind and wise words.


UPDATE
=====

My rebuilt MAF arrived this morning. I measured its resistance (pins 3 & 6), even though that shouldn't matter, at 500 ohms.

For a control test, I started up my 928 cold with the old MAF. This time it started and immediately died. Safe to say that my car was behaving worse, regressively over time.

Changing nothing else, I installed the new/rebuilt MAF. Car starts and runs fine now. I drove it all around town, changed my oil, changed my fuel filter..had a blast.

Life is good.

So yes, it *was* a failing MAF that caused this problem, and the situation is now resolved.

Stromius

Glad to see you're back on the road! My MAF delivery is delayed due to weather here in Seattle but I guess I wouldn't be going anywhere since it's icy on most side roads. Can't wait to see if 88 s4's are 2 for 2 on MAF's this month.